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Expensive portrait lenses with bad bokeh
On 29/07/2016 19:02, Rich A wrote:
Too many 85mm f/1.4 lenses seem to suffer from this. It is unpleasant and imparts a disorienting feeling of motion to the outer field. Oddly, I've seen some cheaper 85mm lenses and older, slower ones that don't have this problem. https://www.dpreview.com/forums/post/58110604 There's an attempted explanation in that thread (post #5): "The off-center "ovals" are caused by internal vignetting -- the fact that the lens has significant optical thickness. The more complex the lens design, the harder it is for the designer to minimize this effect -- an uncorrected single element doesn't really have this problem. If you hold the lens straight ahead and look through it, the opening appears round; however, if you look at an angle, you'll see the circular shape is clipped (typically) as the intersection of two circles of different diameters, and that's exactly how the bokeh ball (technically OOF PSF -- out-of-focus point spread function) gets shaped. If you stop the lens down, eventually the aperture becomes small enough to only let rays pass that are within the unobstructed intersection, and the shape will be that of the aperture across the entire frame." |
#2
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Expensive portrait lenses with bad bokeh
On 30/07/2016 15:08, RichA wrote:
On Friday, 29 July 2016 21:34:07 UTC-4, Me wrote: On 29/07/2016 19:02, Rich A wrote: Too many 85mm f/1.4 lenses seem to suffer from this. It is unpleasant and imparts a disorienting feeling of motion to the outer field. Oddly, I've seen some cheaper 85mm lenses and older, slower ones that don't have this problem. https://www.dpreview.com/forums/post/58110604 There's an attempted explanation in that thread (post #5): "The off-center "ovals" are caused by internal vignetting -- the fact that the lens has significant optical thickness. The more complex the lens design, the harder it is for the designer to minimize this effect -- an uncorrected single element doesn't really have this problem. If you hold the lens straight ahead and look through it, the opening appears round; however, if you look at an angle, you'll see the circular shape is clipped (typically) as the intersection of two circles of different diameters, and that's exactly how the bokeh ball (technically OOF PSF -- out-of-focus point spread function) gets shaped. If you stop the lens down, eventually the aperture becomes small enough to only let rays pass that are within the unobstructed intersection, and the shape will be that of the aperture across the entire frame." Nikon has a new 105mm f/1.4 ($2200) coming. I'm wondering if it'll be patterned after their relatively recent 85mm f/1.4 which has a slightly oval edge effect? Their 58mm f/1.4 has terrible wide-open edge quality (use a background consisting of point-source lights to see). I'm not totally convinced by the explanation given by the poster above, I kind of wonder if rectilinear correction of 50mm lenses also has something to do with the misshapen blur circles, so longer may be better. The 105mm f1.4 at $2,200 - more than I could justify for the limited portrait I do, but I expect it will be very popular with pros. They better have any aberrations well sorted so that PDAF can be fast and flawless when using off-centre AF points. |
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